The present invention relates to an optical communication system that can transmit and receive an optical signal via an optical fiber serving as a transmission medium. More particularly, it relates to an optical communication system that can be used for domestic communication, communication among electronic devices, LAN (Local Area Network) and the like.
With the development of information-oriented society, network technology utilizing an optical fiber has become the center of public attention. In particular, the application of an optical communication system utilizing a plastic optical fiber (hereinafter referred to as POF) as a transmission medium to domestic communication, LAN and communication among electronic devices has been advanced.
FIG. 22 shows an example of an optical communication system of the above type. An optical communication system 103 has an optical fiber 2 for transmitting modulated light suited to transmission, based on a data signal to be transmitted, and optical communication modules 101 connected to both end portions of the optical fiber 2 so as to be optically coupled thereto. The optical communication system shown in FIG. 22 is of a minimum construction, and there is a case where the optical communication system has additional optical communication modules 101 and additional optical fibers 102 so as to constitute a network.
There is also a case where optical communication modules manufactured by different manufacturers (optical communication modules having different optical characteristics), or optical communication modules having different communication speeds coexist in the same optical communication system. Thus, an optical communication system is not necessarily constructed of the same optical communication modules.
Optical communication systems 103 can be classified into several types according to their communication modes. When roughly classified, the following cases are exemplified: (1) cases where one optical fiber 2 is used, or a plurality of optical fibers 2 are used; (2) cases where two-way communication of signals is performed, or one-way communication of signals is conducted; and (3) cases where simultaneous (full-duplex) communication of signals is performed, or semi-duplex communication of signals is performed; and the like. Optical communication is established by a scheme in which these modes are used in combination (for example, two-way full-duplex communication is performed via a single-core optical fiber).
In the optical communication system as above, the transmission distance may vary from 1 m to about 100 m according to its use and layout condition. As an optical fiber 2, for example, a multimode optical fiber such as a POF has been proposed. The POF consists of a core made of a plastic having superior optical permeability, such as PMMA (polymethyl methacrylate) and polycarbonate, and cladding made of plastic having a refractive index lower than that of the core. Such an optical fiber 2 makes it easier to increase the diameter of the core from about 200 μm to about 1 mm, in comparison with a quartz optical fiber. Thus, it is easy to adjust the coupling between an optical communication module 101 and the optical fiber 2, and therefore an inexpensive optical communication system 103 can be obtained. A PCF (polymer clad fiber), of which the core is made of quartz glass, and of which the cladding is made of a polymer, may also be used. Although the PCF is more expensive than the POF, the PCF is characterized by having a smaller transmission loss and having a wider transmission band. Therefore, an optical communication system 103 that can perform communication at a longer distance and at a higher speed can be obtained by using the PCF as a transmission medium.
However, in the case where POFs are used as the transmission media, since their transmission loss is larger (about 0.2 dB/m) than that of quartz optical fibers, a difference in transmission loss due to different transmission distances becomes larger. Thus, there is a problem in that a variation in the quantity of received light becomes larger. An optical communication system having a large variation in the quantity of received light requires an increased dynamic range. This disadvantageously complicates the design of the optical communication system and increases production cost.
As a means of solving the above problems, a method has been proposed in which a variation in the quantity of received light depending on the transmission distance is reduced, and those solutions that follow are known.
(1) When the transmission distance is long, optical amplification is performed in the middle of an optical fiber to increase the quantity of light (e.g., JP-A-148988).
(2) In the case where the quantity of received light is changed, a gain of a receiver is switched by using an auto gain control (AGC).
(3) The quantity of light transmitted by a transmitter itself is changed depending on the length of the transmission distance (e.g., 2000-156664).
However, the method (1) in which optical amplification is performed increases cost, and the wavelengths and systems to be used are limited. Thus, the application of this method is only limited to a main line system, and it is difficult to use this method for a short-distance optical communication system utilizing a transmission medium such as a POF.
(2) When an AGC is used, the circuit becomes complicated. Further, a system for detecting the quantity of received light is required in order to switch the gain, which results in an increase in cost.
(3) Also, when the intensity of transmitted light is varied, a system for detecting the transmission distance or the quantity of transmitted light and performing feedback is required. Thus, there is a problem of an increase in cost.
In the optical communication system, as mentioned above, optical communication modules with different communication speeds, and optical communication modules with different optical characteristics may coexist. In such a case, it has been more difficult to suppress the increase in dynamic range due to a variation of the transmission distance.
Incidentally, JP-A-1-177004 discloses that the numerical aperture of transmitted light of an optical communication module is made smaller than the numerical aperture of an optical fiber, whereby the frequency band of a transmission signal is widened and the communicable distance is extended. However, the publication does not disclose any solution to a problem that the received light quantity is varied depending on the length of the transmission distance and the dynamic range increases. Further, according to the technique disclosed in the publication by which the numerical aperture of the optical communication module is made smaller than that of the optical fiber, if a light-emitting diode (LED), which has a large radiation angle, is used as a light emitting element, the optical system becomes complicated, which makes it difficult to achieve reduction in the size and cost of the optical communication module.